Easy Meg Calibration Using Just 1 Litre of O2

Easy Meg Calibration Using Just 1 Litre of O2
By Skipbreather



There certainly is a wide diversity of opinions expressed here at Rebreather World, but one subject on which there is pretty much universal agreement is the need for accurate sensor calibration. For the Meg, at the final point in the calibration sequence the sensors need to see 100% O2 at ambient pressure. There are lots of ways to get there with some methods using far more O2 than others. The project here was to find a way to do it in a repeatedly reliable way while using only 1 to 2 litres of O2. The bits and pieces shown allow a complete O2 flood, which in turn allows a very accurate calibration, while using just the O2 trapped in the regulator and hoses after the tank valve has been turned off.


The Short Version

To use, first the Meg electronics are turned on and the calibration option is selected (yes, I AM really sure!). The ambient air portion of the calibration sequence is done normally. The plug is then inserted as shown and 100% O2 is supplied to it at a low flow rate. When you are happy with the readings, confirm the calibration.



The Long Version

The first part of the project needed a way to reduce the volume flooded to just a couple of cubic inches. Smaller volume to flood- less gas needed. What I came up with was the one-piece calibration plug first mentioned in a post several weeks ago. It is a VERY simple lathe project which has only two dimensions that need close attention to detail, and would make a good first project for someone who owns a small lathe but is not ready to tackle metal work yet.

The other half of the project was to find an accurate yet simple and inexpensive way to meter O2 to the cal plug at a very low flow rate. This was achieved using squirreled away parts from the leftovers box (hate to call it a junk box!). The parts used consist of BC coupler, a small brass compression fitting, some white plastic icemaker tubing, and the central part, a small plastic orifice.


Let’s Start With The Calibration Plug

I worked my way to this part’s design after starting with my home brew version of the factory calibration kit. The kit certainly saves a lot of O2 over a total loop flush but I had a gut feeling I could do much better. I had one of those “ah-ha!” moments when it finally dawned on me that since the Meg always needs ambient air for the first half of calibration sequence then the bottom of the head will always be exposed. That brought me to the first design point: Flood just the chamber where the cells see the loop gas. This lead to second design point: Reduce even that small volume by filling it as much as possible with the plug. The final design point grew from the second: Extend the plug past the chamber with a fit loose enough to prevent back pressure (which would invalidate the calibration) but tight enough to prevent backflow of ambient air. It works.


The plug pictured extends about ½ inch past the sensor chamber, towards the top of the head. Its diameter is approximately 0.022 inches less than the gas passage at that point. Leon was kind enough to make the diameter of that area and the diameter of the inside of the neck that connects to the scrubber EXACTLY the same diameter (thanks, Leon!) so it made design of the plug easy.


I’m not going to attempt to teach lathe operation here, but I will run through several construction considerations.

Start with about 4 inches of 1.75” diameter type 1 PVC stock. I got mine from McMaster-Carr.

Chuck it up in the 3 jaw and turn it down to 1.100 inches in diameter for the length shown. This diameter is the first important dimension- don’t leave it any bigger, but .002 to .004 undersize won’t hurt anything. Now cut the O-ring grooves where indicated. Exact placement is not critical, but an accurate depth IS. They should be 0.079” deep. Any less, plug won’t fit. If the cut is much more than .002 to .004 too deep, the fit will be too sloppy. Put a small chamfer on the end & cut the plug free from the stub. Cut the plug just a bit long, leaving yourself a small amount of stock to clean up the inevitable bad finish left by the cut-off tool.

Now turn around the plug and put it back into the 3 jaw for the drilling and tapping of the hole as shown. Trick: to avoid marring the newly cut surface, slip a short length of bicycle inner tube over the plug before putting it back into the three jaw. Remove the plug, remove the inner tube sleeve, drill the two 3/16” holes as shown and you’re done. Degrease the part, install the hose barb (3/16” barb X 1/8” mnpt), and you are ready for testing.

Testing is both very simple and very important. Manufacturing tolerances can and do vary and we need to be certain no O2 back pressure is being created.

To test, get your hands on an O2 regulator that will allow you to adjust the lpm flow. I used the medical O2 reg from my rescue kit. Get your electronics to the part of the cal where the MV readouts are on the screen. Install the plug and flow O2 at the sensors at the highest rate available- mine was 15 lpm. Observe the MV readouts once they stabilize and then start reducing the flow one regulator flow setting at a time, noting a stabilized MV reading before reducing it each time. If there is no change in the MV readings in the lowest 2 or 3 flow settings, you have a good plug. Real world example: At 15 lpm my #1 sensor stabilized at 51.4 MV. This dropped to 51.0 MV at 10 lpm. The 51.0 MV was unchanged at 6 lpm, 4 lpm and 2 lpm, indicating essentially no back pressure = good to go on the O2 calibration.


Now On To The Flow Restrictor

With other rigs I’ve always used medical regulators and external bottles of O2 for calibration. I didn’t want to waste the on board gas. That’s how I first used the above plug. I eventually came to realize, however, that the flood volume was SO small, calibration using on board O2 was entirely practical.

In my leftover parts bin were a few small orifices from years ago when I converted an IDA71 to CMF O2 addition. Some were .003”, some were .004”, and all had 1/8” barb input and output. They are made from polycarbonate and have a filter screen built in. And, get this: They cost less than $4 each. They are made by a company called Airtrol.

They do not sell direct, but have sales reps listed on their site. I bought the old fashioned way, from a local distributor via telephone, but one of the sites listed does sell on the web - Coast Pneumatics

The part numbers are 0-003-125 for the .003 orifice and 0-004-125 for .004 orifice. Either will work for this flow restrictor.

The job, then, is to get your O2 from the BC type connector on your manual O2 addition hose to the input barb of your new calibration plug. The parts pictured below get it done at less than $10 total cost.


What you see above is all it takes. From left to right, the orifice, ½ inch of white plastic ice maker tubing, common compression fitting (nut, barrel, and body), and a BC connection adapter. Above them is a short length of clear plastic tubing that connects the orifice to the plug’s input barb.


Some Additional Notes

The orifice only flows in ONE direction (internal filter, remember?). Pay attention to the little arrow. The ice maker tubing is true ¼”. There is some 7mm tubing on the market sold as ¼” that will not work- it’s just a hair too large. The compression end of the fitting needs to be ¼” but the other end only needs to match whatever you have for a BC input barb. I went with ¼” fnpt fitting and a ¼” mnpt-BC barb adapter.


Assembly

Slip the bit of white poly tubing onto the INPUT barb of the orifice.


Then put on the compression nut.


Now slip on the compression barrel.


Assemble the nut to the body. Note: Be careful during the draw up of the compression nut. If the compression barrel slips just a bit the orifice will be a little cocked in the fitting. It will still work fine- it’ll just look a tad strange.


Add the BC adapter and output tubing and you’re done.


Here are some real world test numbers from my bench. Please keep in mind your mileage may vary somewhat.


The .004 orifice is purple. When connected to the BC coupler for the manual O2 hose, it flow checked at 1.2 lpm. Starting from ambient air it took 50 seconds for my sensors to reach a stable MV output. Fully half of that time was spent rising the last 5%. That’s pretty darn close to just 1 liter of gas to calibrate! To give you an idea just how little gas this orifice uses: I attach my O2 reg (the Scubapro MK2 supplied by ISC). Open tank valve. Let reg pressurize. Close tank valve. Plug in orifice. It now takes 2 minutes 15 seconds for the pressure gauge to drop by just 1000 psi.

The .003 orifice is gold/ brown. Its performance is similar to the purple one with the expected variations. It flow checks 0.9 lpm and takes 1 minute 10 seconds to get the stable MV readings, again pretty close to 1 liter of O2. In the bleed down of the system, it took 3 minutes 15 seconds to drop the 1000 psi.


A couple of final notes

1. My sensors are not quite a year old. Depending on the age of yours, response time may be greater or less.
2. Although probably not all that necessary at these pressures, as a precaution all metal parts went through the ultrasonics for O2 cleaning.
3. If something does not seem right, FIND OUT WHY! You’ll live longer.

Ya’ll be careful out there and dive safe!

Ken



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